Method and apparatus for monitoring a redundant (standby) transmitter in a radio communications system

ABSTRACT

A method for monitoring a redundant (passive) transmitter, being, for example, part of a base station of a point-to-multipoint radio communications system transmits, during normal operation, a spread-spectrum signal over the redundant transmitter, the spread-spectrum signal being of low spectral power in comparison with data signals being transmitted by the active transmitter of the base station. One or more receivers are associated, for example, with terminal stations in communication with the base station and detect the presence of the spread-spectrum signal. If the spread-spectrum signal is not found to be present, the receivers provide an indication of this, and from this indication, a decision is made as to the integrity of the redundant transmitter.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for monitoring aredundant (or standby) transmitter in a radio communications systemhaving an active transmitter and one or more receivers. The invention isespecially, although not exclusively, concerned with point-to-multipointradio communications systems.

Radio transmission systems are often used to transmit data. A commonscenario is the transmission of data from a base station to a number ofreceivers in a point-to-multipoint system. Such an arrangement is shownin FIG. 1, in which the base station (BS) communicates with a number ofterminal stations (TS1-TS3) in one particular sector 10 of the basestation's range. To reduce the impact of equipment failure (i.e., toincrease the availability of a radio link) it is known practice toinstall duplicate equipment (standby equipment) that is redundant whilstthe equipment functions correctly. In the case of a point-to-multipointsystem, since the integrity of the base-station transmitter facility isof prime importance, it is that transmitter that is normally duplicated.This is illustrated in FIG. 2, in which the active and passive(redundant or standby) transmitters are basically identical and comprisean indoor part (IDU) and a cable-linked outdoor part (ODU). The ODUincludes an RF stage and an antenna 11. Where failure of the activetransmitter occurs, the redundant (standby) transmitter can take overthe transmission of data.

There is the possibility, however, that the redundant transmitter willfail before the active transmitter fails. This is especially likely whenthe redundant transmitter is permanently energised (so-called “hotstandby”). Failure then has a probability of 50%. If failure of theredundant transmitter is not detected, the redundancy is lost and, inthe event that the active equipment also fails, data transmission iscompletely lost. This is obviously particularly disastrous inpoint-to-multipoint systems, since a whole sector can be lost with thefailure of the base station.

There is, therefore, a need to monitor the integrity of the redundanttransmitter in such a communications system.

The supervision of a redundant transmitter, however, poses an acuteproblem, as a transmitter can only be fully tested by transmitting asignal, which in turn may adversely affect the transmission of data.

At the present time, three methods of supervision are known:

-   -   the data signal uses both transmitters (active and redundant) at        different frequencies or time slots. This is possible only in        point-to-multipoint systems if the data signal in the downlink        is “bursty”, i.e. if different time slots are employed for        different terminal stations, or if it is an FDM        (Frequency-Division Multiplex) signal, i.e. different frequency        bands are used for different terminal stations.    -   a pilot signal is transmitted over the redundant transmitter,        either in the time or frequency domain. This is, however, a        waste of resources.    -   the outdoor unit (ODU) is self-monitoring, i.e. the supervision        is performed only at the analogue part of the transmitter, for        example by monitoring the oscillators. This requires a special        development of the redundant ODU and a digital link between the        redundant ODU and IDU (indoor unit). Further, this involves        additional hardware outlay, which can be costly.

Since in the case of a TDMA (Time-Division Multiple Access)point-to-multipoint system the downlink signal is time-continuous, thefirst method for supervision cannot be used. The other two have thedrawback that they are inefficient.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided amethod for monitoring the redundant transmitter of a radiocommunications system, which system comprises an active transmitter forthe transmission of data signals to one or more receivers, the methodbeing characterised by the steps of: during normal operationtransmitting over the redundant transmitter a spread-spectrum signalwithin the frequency band of the data signal, said spread-spectrum (SS)signal being of low spectral power in comparison with said data signals;and at one or more of the one or more receivers, detecting the presenceof the spread-spectrum signal, the absence thereof being taken toindicate the non-integrity of the redundant transmitter.

The invention provides the advantage that a pilot (or monitoring) signalcan be transmitted over the redundant transmitter without disturbing thedata signal being transmitted by the active transmitter. At thereceiver, as far as the data signal is concerned the SS signal isregistered merely as additional white noise and is therefore negligible.The receiver, however, also contains means for specifically detectingthe SS signal and, if it is not present, it can be concluded that theredundant transmitter is defective.

A particular advantage of the invention is its universal applicability(it does not depend on the nature of the data signal), the fact that itdoes not waste time or frequency resources, and its susceptibility todigital realisation, so that it is also cost-effective. Further, the ODUdoes not have to be specially designed and no measurement setup or linkbetween the ODU and the IDU has to be provided.

Preferably detection for the presence of the spread-spectrum signal isperformed by a process of correlation.

Advantageously detection is performed by a process of cross-correlation,the spread-spectrum signal being provided by feeding the redundanttransmitter with a first pseudo-noise signal and the cross-correlationbeing performed between the received signal and a second pseudo-noisesignal, the second pseudo-noise signal having the same characteristicsas the first. With such a method the first pseudo-noise signal ispreferably passed through the same components in the redundanttransmitter as would data signals, were the redundant transmitter calledupon to take over from the active transmitter.

Advantageously, and in order to compensate for a timing phase error inrespect of the spread-spectrum signal, oversampling of the receivedsignal is performed in the receiver.

Preferably, and in order to compensate for the effects of a frequencyoffset existing between the active and redundant transmitters and totake into account the narrow allowable window of offset which thecorrelation of a long pseudo-noise sequence can tolerate, the receivedsignal is subjected to a stepped sweeping operation, wherein thereceived function is multiplied by a complex factor having the formexp{j2πkδ_(S)/η}, where j=√−1, k is the sampling index, η is anoversampling factor and δ_(S) are the sweeping steps, scaled by thesymbol rate.

Preferably the sweeping steps δ_(S) are chosen such as to cover allvalues of the frequency offset and are advantageously chosen such as tocompensate for a drift of the frequency offset with time.

Preferably the cross-correlation is two-dimensional, and is calculatedfor all sweeping steps δ_(S) and for ηN_(pn) time steps, where η is theoversampling factor and N_(pn) is the length of the pseudo-noisesequence. Advantageously a maximum of the absolute value of thecross-correlation result is employed to determine the integrity of theredundant transmitter.

In one embodiment a maximum of the squared absolute value of thecross-correlation result is employed to determine the integrity of theredundant transmitter.

Preferably the correlation calculation is performed by a part-serial,part-parallel processing of the sampled data. With such a method theprocessing preferably takes the form of a processing of a first group(N_(S)) of the ηN_(pn) points in parallel for successive values ofδ_(S), then of a second group (N_(S)) of the ηN_(pn) points in parallelfor successive values of δ_(S), and so on until all ηN_(pn) points havebeen covered.

In a preferred application of the method of the present invention theradio communications system comprises two or more receivers, each ofwhich provides an indication of the presence or absence of thespread-spectrum signal, the decision as to the non-integrity of theredundant transmitter being taken on the basis of the indications of apredetermined number of the two or more receivers.

Advantageously the decision is taken on the basis of a majority vote.

Alternatively the radio communications system comprises one receiver andthe indication of non-detection of the spread-spectrum signal at thatreceiver is taken as an indication of the non-integrity of the redundanttransmitter.

The present invention finds particular application topoint-to-multipoint radio communications system in which the active andredundant transmitters are part of a base station, and the receivers areterminal stations, of that point-to-multipoint system. Preferably thepoint-to-multipoint system comprises a TDMA (Time-Division MultipleAccess), a FDMA (Frequency-Division Multiple Access) or a CDMA(Code-Division Multiple Access) system.

Alternatively the invention can be applied to point-to-point systemsincorporating a redundant or standby transmitter.

According to a second aspect of the invention there is providedapparatus for monitoring the redundant transmitter of a radiotransmission system, which system comprises an active transmitter forthe transmission of data signals to one or more receivers, the apparatuscomprising: means for generating a pseudo-noise signal; means forapplying said pseudo-noise signal to an input of the redundanttransmitter, the transmitter thereby transmitting a spread-spectrumsignal having a low spectral power in comparison with said data signals;and means in one or more of the one or more receivers for detecting thepresence of the spread-spectrum signal.

Advantageously one or more receivers includes correlator means for thecross-correlation of the received signal.

Preferably the one or more receivers further comprises oversamplingmeans for the compensation of a timing phase error in respect of thespread-spectrum signal.

Advantageously the apparatus further comprises sweeping means forsubjecting the received signal to a frequency-sweeping operation, thesweeping means comprising a multiplier means for multiplying thereceived signal by a complex factor having the form exp{j2πkδ_(S)/η},where j=√−1, k is the sampling index, η is an oversampling factor andδ_(S) are the sweeping steps, scaled by the symbol rate.

Preferably the correlator means is connected to a maximum-deriving meansfor deriving a maximum value of the correlator output. With such anarrangement the maximum-deriving means is advantageously arranged toderive the maximum of the absolute value of the correlator output or themaximum of the square of the absolute value of the correlator output.

According to a further aspect of the invention a radio communicationssystem comprises an active and a redundant transmitter, two or morereceivers and an apparatus in accordance with the second aspect of theinvention.

Preferably the monitoring apparatus of such a communications systemcomprises a decision-making means fed by the indications ofredundant-transmitter integrity delivered by the receivers. Preferablythe decision-making means makes a decision on the basis of majorityvoting among the receivers.

The communications system preferable comprises a point-to-multipointsystem such as a TDM/TDMA (Time Division Multiplex in a downlink/TimeDivision Multiple Access in an uplink direction) system, an FDM/FDMA(Frequency Division Multiplex in a downlink/Frequency Division MultipleAccess in an uplink direction) system or a CDM/CDMA (Code DivisionMultiplex in a downlink/Code Division Multiple Access in an uplinkdirection) system.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, by way of exampleonly, with reference to the drawings, of which:

FIG. 1 is a simplified diagram of a point-to-multipoint radiocommunications system;

FIG. 2 shows the use of an active and passive (redundant) transmitter ina base station of the point-to-multipoint system of FIG. 1;

FIG. 3 is a block diagram of a radio communications system employing themonitoring method in accordance with the invention;

FIG. 4 is a complex-plane diagram illustrating the action of the mappershown in FIG. 3;

FIG. 5 is an equivalent model of the communications system of FIG. 3;

FIG. 6 is a sin(x)/x diagram showing the effect of the presence of afrequency offset between the active and redundant transmitters;

FIG. 7 is the equivalent model of FIG. 5, but including a sweepingtechnique for compensating for the effects of frequency offset;

FIG. 8 is a further illustration of the sweeping technique mentioned inconnection with FIG. 7;

FIGS. 9 and 10 are, respectively, a diagram illustrating the deleteriouseffect of frequency-offset drift and a method of compensating for thiseffect;

FIG. 11 is a block diagram depicting the correlation process that occursin an embodiment of the invention;

FIG. 12 shows a method of correlation which involves part-serial,part-parallel processing of data;

FIG. 13 is the block diagram of FIG. 11, expanded to include theserial/parallel correlation procedure illustrated in FIG. 12, and

FIG. 14 is a diagram illustrating a particular structural realisation ofthe serial/parallel correlation technique.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 3, FIG. 3 is a block diagram of a radio linkfeaturing the monitoring method of the invention and includes bothactive and passive (redundant/standby) paths in the transmitter and thereceiver. The transmission stages are the same in both paths and includea mapping stage 12, an interpolation stage 13, a digital-to-analogueconverter stage 14 and an IF/RF output stage 15 which feeds an antenna16. The modulation scheme which is used is a linear one, e.g. quadratureamplitude modulation (QAM).

Describing the function of the illustrated components in a little moredetail, the bits (data symbols) which are to be transmitted are firstmapped into channel symbols in the mapper 12. Assuming, for example,that a 4QAM or a QPSK (Quadrature Phase Shift Keying) modulation schemeis employed, the channel symbols are made to correspond to one of thefour points in the complex plane shown in FIG. 4, i.e. two bits of thewanted data bit stream (symbol stream) to be transmitted are mapped ontoone of these four symbols, yielding for that symbol two complex values(real and imaginary) at the output of the mapper. There then occurs apulse-shaping process using a square-root Nyquist filter (interpolationstage) 13. The output of this filter 13 is sampled and converted intoanalogue form (14). In order to satisfy the sampling theorem, thesampling rate must be at least twice the channel symbol rate, andtherefore an oversampling factor-of-η interpolation function isperformed, which is included in the pulse-shaping block (interpolationstage) 13. Finally, the baseband signal is up converted to IF and thento RF (15) before being transmitted from the antenna 16.

The data signal is transmitted over the active path.

While the redundant transmission path is substantially identical to theactive path in all hitherto described respects, in one respect it isdifferent: at the input of the mapper 12 a switch 17 is provided whichmakes it possible during normal operation to feed in a spread-spectrum(SS) signal at symbol rate, but, following failure of the activetransmitter, to take over transmission of the data signal.

The SS signal consists of a long pseudo-noise (PN) sequence (±1 inamplitude) of length N_(pn). The signal is up sampled with anoversampling factor η. As the digital filters and analogue components inthe active and passive line card and ODU are essentially the same, theSS signal will have the same bandwidth as the data signal. However atthe air interface its spectral density will be significantly lower thanthe density of the data signal (about 30-35 dB lower). Thus, for thedata signal the PN sequence appears only as an additional, butnegligible, white noise.

At the receiver side the signal received at a receiver antenna 18 isdown converted again to baseband (19) and changed from analogue form todigital (20), following which a corresponding interpolation step isperformed (21), the resulting signal being, in accordance withconventional methods, subject to a synchronisation and equalisationprocess (22), thereby producing the data signal originally transmittedby the active transmitter. In addition to this, however, steps are takento detect the SS signal that is being transmitted by the redundant(passive) transmitter. This is achieved by correlating the receivedsignal with the same PN sequence that was used by the transmitter. Thisis shown by the separate branch 23, which is taken to a correlationstage 24, the result of the correlation process being used to make adecision as to whether or not the SS signal was received. (It is notedthat the received signal comprises both the transmitted data signal andthe SS signal). In practice it is the maximum of the absolute value ofthe correlation which is used to form the decision.

The decision stage (24) draws the following conclusions:

(a) where both data and SS signals are registered, the ODUs (11, 15, 16)of both active and passive transmitters are deemed to be intact;

(b) where the data signal only is registered, the redundant ODU isconsidered to be defective;

(c) where the SS signal only is registered, the active ODU is taken tohave failed (this applies to the time just before redundancy switchingtakes place). The switching over of control from active unit toredundant unit is not addressed by this present patent.

This simple approach is rendered more complex by the need to solve twoproblems which have been found to arise in a practical system:

(1) The frequency synchronisation which can normally be employed in thecase of the data signal is missing in the case of the SS signal. Thiscauses a degradation of the correlator output of the form sin(πδM/η/(πδM/η) where δ is the frequency offset scaled by the symbolrate, M is the correlation length and η is the oversampling factor.(2) There is a similarly missing timing synchronisation for the SSsignal, which also causes a degradation of the correlator output,depending on the oversampling factor: the larger the factor η, thesmaller the degradation.

Even if a frequency and timing phase synchronisation is provided for theradio link, it is optimised for the active path only. As the oscillatorsin the active and passive IDU and ODU are not coupled, the frequencyoffsets will not be the same. Furthermore, the radio channels for thedata and SS signal will not be the same. The sampling phase at thereceiver will be optimised for the data signal, so that there can be atiming phase error for the SS signal.

The impact of frequency offset and timing phase error on the correlationis now examined and the solution presented. A baseband signalrepresentation is assumed.

The effect of a frequency offset in the time domain is a phase rotationof every sample by a constant factor

$2\pi\frac{\;\delta}{\eta}$with respect to the previous sample:

$\begin{matrix}{s_{k}^{\prime} = {{s_{k} \cdot {\exp\left( {{j \cdot 2}{\pi \cdot k}\;\frac{\Delta f}{f_{A}}} \right)}} = {s_{k} \cdot {\exp\left( {{j \cdot 2}{\pi \cdot k}\;\frac{\delta}{\eta}} \right)}}}} & (2.1)\end{matrix}$where η=f_(A)/f_(S) is the oversampling factor, δ=Δf/f_(S) is thefrequency offset Δf scaled to the symbol rate f_(S) and f_(A) is thesampling rate.

Referring to FIG. 5 there is shown an equivalent model of thecommunications system of FIG. 3 in which r_(k) and s_(k) are the datasignal and the SS signal respectively, both oversampled with η. Leth_(A) and h_(P) be the impulse responses of the whole active and passivetransmission path, respectively, and δ_(A) and δ_(P) the resultingfrequency offsets scaled to the symbol rate. The simplification ofconcentrating all filters at the beginning of the transmission path andall frequency offsets at the end is based on the fact that filtering andfrequency offset can be interchanged if the filter bandwidth of thereceiver filter is large compared to the frequency offset and if thefrequency offset is small compared to changes of the filter function inthe frequency domain.

Let M be the correlation length and k_(o) an arbitrary starting index.The output φ_(ys)(n) of the correlator is:

$\begin{matrix}{{\varphi_{ys}(n)} = {\frac{1}{M} \cdot {\sum\limits_{k = k_{0}}^{k_{0} + M - 1}y_{k \cdot s_{k - n}^{*}}}}} & (2.2)\end{matrix}$

Expressing y_(k) by convolution we have:

$\begin{matrix}{{\varphi_{ys}(n)} = {{\sum\limits_{l}^{\;}\;{h_{pl} \cdot \frac{1}{M} \cdot {\sum\limits_{k = k_{0}}^{k_{0} + M - 1}\;{s_{k - 1} \cdot s_{k - n}^{*} \cdot {\exp\left( {{j \cdot 2}{\pi \cdot k}\frac{\delta_{p}}{\eta}} \right)}}}}} + {\sum\limits_{l}^{\;}\;{h_{Al} \cdot \frac{1}{M} \cdot {\sum\limits_{k = k_{0}}^{k_{0} + M - 1}{r_{k - 1} \cdot s_{k - n}^{*} \cdot {\exp\left( {{j \cdot 2}{\pi \cdot k}\frac{\delta_{A}}{\eta}} \right)}}}}}}} & (2.3)\end{matrix}$

The mean value of the correlator output is therefore (assuming thats_(k) and r_(k) are uncorrelated):

$\begin{matrix}{{E\left\lbrack {\varphi_{ys}(n)} \right\rbrack} = {\sigma_{s}^{2} \cdot h_{p_{n}} \cdot \frac{1}{M} \cdot {\sum\limits_{k = k_{0}}^{k_{0} + M - 1}{\exp\left( {{j \cdot 2}{\pi \cdot k}\frac{\delta_{p}}{\eta}} \right)}}}} & (2.4)\end{matrix}$σ_(S) ² is the variance (or power) of the SS signal. Transforming thesum and considering only the absolute value, we have finally:

$\begin{matrix}{{{E\left\lbrack {\varphi_{ys}(n)} \right\rbrack}} = {\sigma_{s}^{2} \cdot {h_{p_{n}}} \cdot {{{si}\left( {\pi\frac{\delta_{p}}{\eta}M} \right)}}}} & (2.5)\end{matrix}$where si(x)=sin(x)/x. This is a very important result. It shows that,when a frequency offset is present, the “usual” correlator output σ_(S)²h_(P) _(n) is distorted by a si-function of the product

$\frac{\delta_{p}}{\eta}M$(see also FIG. 6). Obviously, for large M, si

$\left( {\frac{\delta_{p}}{\eta}M} \right)$will be close to zero and the correlation φ_(ys) will become very small.In this case, the SS signal would not be detected. However, there is aregion for δ_(P) where detection is possible, i.e. where the degradationcan be tolerated. Unfortunately, the correlation length has to be quitelarge in order to detect the SS signal with its very low power, so theacceptable δ_(P) is too small.

The proposed solution according to the invention is to carry out a“sweeping” process, where an intentional and stepwise changing frequencyoffset δ_(S) is introduced before the correlator. The correlation iscalculated for a number N_(d) of offsets, thus covering the whole rangeof δ_(P). FIG. 7 reproduces the equivalent model of FIG. 5, but thistime with the additional sweeping function. It should be noted that thecorrelator output φ_(ys)(n, δ_(S)) is now two-dimensional, being afunction of η and δ_(S).

Let δ₀ be the frequency offset with acceptable degradation. If the stepsize is 2 δ₀, then one of the resulting offsets |δ_(p)+δ_(S)(i₀)|≦δ₀ sothat the degradation of E[φ_(ys)(n, δ_(S)(i₀))] will be sufficientlysmall (see FIGS. 6 and 8).

The sweeping function is illustrated in graphical form in FIG. 9. Herethe full range of discrete offsets, δ₁ . . . δ₆ (it is assumed in thisexample that N_(d)=6), is applied in turn, each offset being effectivefor an actual frequency offset of ±δ₀ about that applied offset. Eachδ_(S) is applied for a time Δt₀, this being the time over whichcorrelation takes place for that value of δ_(S).

The effect of this offset compensation can be illustrated also by anumerical example. Assume δ_(P) covers a range from −5 to +5, then thesweeping steps must also vary from −5 to +5. If δ₀=0.5, the step size is1 and δ_(S)(i) assumes the values −5, −4, −3, . . . 3, 4, 5. So if, forexample, δ_(P) has an actual value of 3.2, then the resulting offset forthe particular value of δ_(S)(i₀)=−3 is −0.2, which is (taking theabsolute value) smaller than S₀. If δ_(P)=3.5, the resulting offset willbe −0.5, which is still within the desired range. If δ_(P)=3.6 andδ_(S)(i) is still −3, the resultant offset will be −0.6, which is nowtoo great; hence the correct value of δ_(S) will in this case be −4,yielding an acceptable resultant offset value of −0.4.

In addition to the frequency offset, there is another effect ofnon-ideal oscillators: the frequency drift. The output of theoscillators not only has an offset Δf, but this offset is also changingin time (drifting). It is:

$\begin{matrix}{{\Delta\overset{.}{f}} = {\frac{{\partial\Delta}\; f}{\partial t} = {f_{s} \cdot {\overset{.}{\delta}}_{p}}}} & (2.6)\end{matrix}$

This effect is illustrated in FIG. 9 by the inclusion of two particularvalues of actual frequency offset δ_(P1) and δ_(P2). The actual offsetwithout drift, 30, is, as might be expected, a horizontal line, whereaswith drift the same characteristic assumes a gradient; this is the line31. As shown, line 31 passes through region 32, which means that offsetis being compensated for. However, line 33 shows another possiblecharacteristic in which, because of drift, no region is being passedthrough, neither region 32 nor region 34. Under such circumstancesfrequency offset would remain uncompensated.

In order, in this situation, to “catch” the offset in one sweepexcursion from −δ_(max) . . . δ_(max), the invention provides for thesweeping steps to be adapted so that the regions δ_(S)(i)−δ₀ . . .δ_(S)(i)+δ₀ overlap, as shown now in FIG. 10. The overlapping Δδ₀ has tosatisfy the inequality:

$\begin{matrix}{{\overset{.}{\delta}}_{p} = {\frac{\Delta\overset{.}{f}}{f_{s}} \leq \frac{{\Delta\delta}_{0}}{\Delta\; t_{0}}}} & (2.7)\end{matrix}$where, as already mentioned, Δt₀ is the time needed to calculate φ_(ys)(n, δ_(S)(i)).

For examining the effect of a timing phase error on the output of thecorrelation receiver only the redundant path is of importance. Thefrequency offset is assumed to be zero. From FIG. 5, and ignoring z_(k),we may write the received signal as a time-continuous function:

$\begin{matrix}{{y(t)} = {\sum\limits_{k = {- \infty}}^{\infty}\;{s_{k} \cdot {h_{p}\left( {t - {k \cdot T_{A}}} \right)}}}} & (2.8)\end{matrix}$

If y(t) is sampled with sampling time T_(A) and phase error τ, itfollows that:

$\begin{matrix}{y_{n} = {{y\left( {{n \cdot T_{A}} + \tau} \right)} = {{\sum\limits_{k = {- \infty}}^{\infty}{s_{k} \cdot {h_{p}\left\lbrack {{\left( {n - k} \right) \cdot T_{A}} + \tau} \right\rbrack}}} = {\sum\limits_{l = {- \infty}}^{\infty}{s_{n - l} \cdot {h_{p}\left( {{l \cdot T_{A}} + \tau} \right)}}}}}} & (2.9)\end{matrix}$

From (2.9) we can see that the timing phase error leads to a modifiedimpulse response of the transmission channel:h′ _(P1) =h′ _(P)(lT _(A))=h _(P)(lT _(A)+τ)  (2.10)h_(P1) is the discrete impulse response of the passive transmissionpath, including all filters from the square-root Nyquist filter 13 atthe transmitter to the similar filter 21 at the receiver (see FIG. 3).

The phase error appears to behave like a sampling phase error with thediscrete-time representation of h_(P)(t). We can therefore take it intoaccount by making all calculations using h′_(P1) instead of h_(P1). Themean value of the correlator output will be:|E[φ _(ys)(n)]|=σ_(S) ² ·|h _(P)(n·T _(A)+τ)|  (2.11)

Note, that the maximum phase error is τ_(max)=T_(A)/2, so we maydecrease τ by increasing the sampling rate. i.e. η.

As has been described above, in order to handle the frequency offset,the cross correlation of the received signal and the PN sequence has tobe calculated for N_(d) sweeping points, in addition to the ηN_(pn)“time” points. Thus, the correlation function is two-dimensional:φ_(ys)(−n, δ_(S)(i))=φ(n, i), n=0, . . . ηN_(pn)−1, i=1, . . . N_(d).For this section, η=2 is assumed. FIG. 11 shows the principle of thecorrelation unit: the received signal y_(k+k0) is rotated by a complexfactor exp(jπδ_(S)(i)k) and then multiplied by the over-sampled outputof the same shift register, as in the transmitter. M values at a timeare accumulated (M is the correlation length), the division by M givingthe cross correlation:

$\begin{matrix}{{\varphi\left( {n,i} \right)} = {{\frac{1}{M} \cdot {\sum\limits_{k = 0}^{M - 1}\;{y_{k_{0} + k} \cdot s_{k_{0} + k + n}^{*} \cdot {\exp\left\lbrack {j \cdot \pi \cdot {\delta_{s}(i)} \cdot k} \right\rbrack}}}} = {\varphi_{ys}\left\lbrack {{- n},{\delta_{s}(i)}} \right\rbrack}}} & (2.12)\end{matrix}$

There are two main ways of calculating φ(n, i): all serial or allparallel. All serial means that the points in the two dimensional space(n, i) are calculated one after another. All parallel means that allvalues of φ(n, i) are calculated at once. The all-serial method requiresthe least outlay in terms of hardware, but is slow; the all-parallelmethod is fast, but incurs greater hardware outlay. The preferredembodiment of the invention employs a compromise solution, in whichcalculations are carried out in a partly serial, partly parallel manner.This brings with it a trade-off between speed and outlay.

The scheme actually envisaged by the invention is shown schematically inFIG. 12. A first block of N_(S) values of n is taken (in the exampleshown, N_(S)=8) and all N_(d) values of i are calculated successivelyfor this block. This is shown by the arrow A. Then the next block ofN_(S) time steps follows, in which again all N_(d) values of i arecalculated one after the other; this is the arrow B. The processcontinues until all N_(pn) values of n have been covered.

This serial/parallel scheme necessitates an amendment to the correlationcalculation diagram shown in FIG. 11. In the amendment (see FIG. 13) thestages between the δ_(S) rotation operator 25 and the maximum-valueblock 26 are duplicated, one for each value of δ_(S). Hence there areN_(S) blocks altogether, each fed from the rotation operator 25 andfeeding the maximum-value block 26. The latter detects which of theunits 1 . . . N_(S) is outputting the greatest absolute value.

A more detailed realisation of this same scheme is shown in FIG. 14,where again N_(S)=8. The figure illustrates the calculation of the firstblock (n=0, . . . N_(S)−1) for δ_(S)(i). The shift register issynchronised with the symbol time T_(S)=1/f_(S) and cycles continuouslywith period N_(pn). For the calculation of the first N_(S) time steps,the first N_(S)/2 (i.e. 4) values of the shift register are read out.

Since the PN sequence is oversampled by η=2, every second value of s_(k)is zero and does not have to be multiplied by r_(k). To take this intoaccount, switches are provided before the accumulators, which aresynchronised by the sampling time T_(A)=T_(S)/2, where T_(S) is thesymbol time. After a time MT_(A) the contents of the N_(S) accumulatorsare divided by M, giving the correlations. Out of every N_(S)correlation values the maximum of their squared absolute value iscalculated (alternatively, the absolute value alone may be calculated,but its square has the advantage of incurring less hardware outlay) andcompared with the stored maximum of the previous sequence of N_(S)correlation values. The larger value is then kept as the new maximum.The accumulators are set to zero and δ_(S) takes on the next value.After t=MT_(A)N_(d), δ_(S) again assumes its first value and the nextblock of N_(S)/2 outputs of the shift register is read out. By comparingthe maximum of all correlator outputs (i.e. their squared absolutevalue) with a given threshold, a decision can be formed as to whetherthe SS signal has been sent or not.

The described principle of monitoring a redundant transmitter by a SSsignal can be used for point-to-point systems as well as forpoint-to-multipoint systems. However, in point-to-multipoint systems thefollowing additional and beneficial feature can be introduced. At eachterminal within a sector the correlation and detection unit describedabove is provided. Each terminal makes a decision as to whether the SSsignal is present or not and the decision is transmitted to the basestation. Only if a predetermined number of terminals indicated that theSS signal had not been received is an alarm then given to the networkmanagement system. Preferably the alarm is only given where at leasthalf of all the terminals gave a negative report, i.e. majority voting.Such averaging over all the terminals allows the requirements of thecorrelator in each terminal to be relaxed (the correlation length may bereduced, for example), without reducing the reliability of thesupervision.

1. A method of monitoring a redundant transmitter of a radiocommunications system having an active transmitter for transmitting datasignals to at least one receiver, the method comprising the steps of: a)during normal operation, transmitting over the redundant transmitter aspread-spectrum signal within a frequency band of the data signals, saidspread-spectrum signal being of low spectral power in comparison withsaid data signals; and b) at said at least one receiver, detecting apresence of the spread-spectrum signal, an absence of thespread-spectrum signal being taken to indicate non-integrity of theredundant transmitter; c) wherein the detecting step is performed by aprocess of cross-correlation, the spread-spectrum signal being providedby feeding the redundant transmitter with a first pseudo-noise signal,and the cross-correlation being performed between a received signal anda second pseudo-noise signal, the second pseudo-noise signal having thesame characteristics as the first pseudo-noise signal, the firstpseudo-noise signal passing through the same components in the redundanttransmitter as would the data signals, were the redundant transmittercalled upon to take over from the active transmitter.
 2. The method asclaimed in claim 1, wherein, in order to compensate for a timing phaseerror in respect of the spread-spectrum signal, the step of oversamplingof the received signal is performed in the at least one receiver.
 3. Themethod as claimed in claim 2, wherein, in order to compensate for theeffects of a frequency offset existing between the active and redundanttransmitters and to take into account a narrow allowable window ofoffset which the correlation of a long pseudo-noise sequence cantolerate, the received signal is subjected to a stepped sweeping step,wherein a received function is multiplied by a complex factor having theform exp{j2πkδ_(S)/η}, where k is a sampling index, η is an oversamplingfactor, and δ_(S) are sweeping steps, scaled by a symbol rate.
 4. Themethod as claimed in claim 3, wherein the sweeping steps are chosen suchas to cover all values of the frequency offset.
 5. The method as claimedin claim 4, wherein the sweeping steps are chosen such as to compensatefor a drift of the frequency offset with time.
 6. The method as claimedin claim 5, wherein the cross-correlation is two-dimensional, beingcalculated for all sweeping steps δ_(S) and for ηN_(pn) time steps,where η is the oversampling factor, and N_(pn) is a length of thepseudo-noise sequence.
 7. The method as claimed in claim 6, wherein amaximum of an absolute value of a cross-correlation result is employedto determine the integrity of the redundant transmitter.
 8. The methodas claimed in claim 6, wherein a maximum of a squared absolute value ofa cross-correlation result is employed to determine the integrity of theredundant transmitter.
 9. The method as claimed in claim 6, wherein acorrelation calculation is performed by a part-serial, part-parallelprocessing of the sampled data.
 10. The method as claimed in claim 9,wherein said processing includes processing a first group (N_(S)) of theηN_(pn) points in parallel for successive values of δ_(S), thenprocessing a second group (N_(S)) of the ηN_(pn) points in parallel forsuccessive values of δ_(S), and so on until all ηN_(pn) points have beencovered.
 11. The method as claimed in claim 10, wherein the radiocommunications system comprises at least two receivers, each of whichprovides an indication of the presence or absence of the spread-spectrumsignal, the decision as to the non-integrity of the redundanttransmitter being taken on the basis of the indications of apredetermined number of the at least two receivers.
 12. The method asclaimed in claim 11, wherein the decision is taken on the basis of amajority vote.
 13. The method as claimed in claim 11, wherein the radiocommunications system comprises one receiver, and the indication ofnon-detection of the spread-spectrum signal at that receiver is taken asan indication of the non-integrity of the redundant transmitter.
 14. Themethod as claimed in claim 11, wherein the radio communications systemis a point-to-multipoint system, the active and redundant transmittersbeing part of a base station, and the receivers being terminal stations,of that point-to-multipoint system.
 15. The method as claimed in claim14, wherein the point-to-multipoint system is one of a group comprising:a TDM/TDMA system, an FDM/FDMA system and a CDM/CDMA system.
 16. Anapparatus for monitoring a redundant transmitter of a radio transmissionsystem having an active transmitter for transmitting data signals to atleast one receiver, the apparatus comprising: a) means for generating afirst pseudo-noise signal; b) means for applying said first pseudo-noisesignal to an input of the redundant transmitter, the redundanttransmitter thereby transmitting a spread-spectrum signal having a lowspectral power in comparison with said data signals; c) means in said atleast one receiver, for detecting a presence of the spread-spectrumsignal; and d) correlator means in the at least one receiver, forcross-correlation of a received signal, the cross correlation beingperformed between a received signal and a second pseudo-noise signal,the second pseudo-noise signal having the same characteristics as thefirst pseudo-noise signal, the first pseudo-noise signal passing throughthe same components in the redundant transmitter as would the datasignals, were the redundant transmitter called upon to take over fromthe active transmitter.
 17. The apparatus as claimed in claim 16,comprising a sweeping means for subjecting the received signal to afrequency-sweeping operation, the sweeping means comprising a multipliermeans for multiplying the received signal by a complex factor having theform exp{j2πkδ_(S)/η}, where k is a sampling index, η is an oversamplingfactor, and δ_(S) are sweeping steps, scaled by a symbol rate.
 18. Theapparatus as claimed in claim 17, wherein the correlator means isconnected to a maximum-deriving means for deriving a maximum value of acorrelator output.
 19. The apparatus as claimed in claim 18, wherein themaximum-deriving means is operative to derive a maximum of an absolutevalue of the correlator output.
 20. The apparatus as claimed in claim18, wherein the maximum-deriving means is operative to derive a maximumof a square of an absolute value of the correlator output.
 21. Theapparatus as claimed in claim 16, and comprising an oversampling meansin the at least one receiver, for compensation of a timing phase errorin respect of the spread-spectrum signal.
 22. A radio communicationssystem, comprising: an active transmitter; a redundant transmitter; atleast two receivers for receiving data signals transmitted by the activetransmitter; and an apparatus for monitoring the redundant transmitter,the apparatus including: a) means for generating a first pseudo-noisesignal; b) means for applying said first pseudo-noise signal to an inputof the redundant transmitter, the redundant transmitter therebytransmitting a spread-spectrum signal having a low spectral power incomparison with said data signals; c) means in said at least tworeceivers, for detecting a presence of the spread-spectrum signal; andd) correlator means in the at least one receiver, for cross-correlationof a received signal, the cross correlation being performed between areceived signal and a second pseudo-noise signal, the secondpseudo-noise signal having the same characteristics as the firstpseudo-noise signal, the first pseudo-noise signal passing through thesame components in the redundant transmitter as would the data signals,were the redundant transmitter called upon to take over from the activetransmitter.
 23. The system as claimed in claim 22, wherein themonitoring apparatus comprises a decision-making means fed byindications of redundant-transmitter integrity delivered by thereceivers.
 24. The system as claimed in claim 23, wherein thedecision-making means makes a decision on the basis of majority votingamong the receivers.
 25. The system as claimed in claim 24, wherein thesystem is a point-to-multipoint system.
 26. The system as claimed inclaim 25, wherein the system is one of a group comprising: a TDM/TDMAsystem, an FDM/FDMA system and a CDM/CDMA system.
 27. The system asclaimed in claim 22, and comprising an oversampling means in the atleast one receiver, for compensation of a timing phase error in respectof the spread-spectrum signal.